Hydrogen and methanol fuel cells are of considerable importance in the search for new energy technologies, see for example, Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. Vol 12A, pp. 55ff, VCH, New York, 1989. One approach in the development of these fuel cells is to employ solid polymer electrolyte membranes in combination with a catalyst layer and a gas diffusion backing (GDB) layer to form a membrane electrode assembly (MEA). The catalyst layer typically includes a finely divided metal such as platinum, palladium, or ruthenium, or a combination of more than one metal such as platinum-ruthenium, or a metal oxide, such as ruthenium oxide, usually in combination with a binder. In hydrogen fuel cells, the catalyst is normally supported on carbon; in methanol fuel cells, the catalyst is normally unsupported. The gas diffusion backing is typically a highly porous carbon sheet or fabric. See Yeager et al, U.S. Pat. No. 4,975,172 for an example of such cells.
A common problem of hydrogen and direct methanol fuel cells is susceptibility to flooding by excessive water which introduces mass transport limitations in the reactant and/or product streams, and thereby disrupts the performance of the fuel cell. It has become common practice to incorporate fluoropolymers in the catalyst layer and gas diffusion backing to impart a degree of hydrophobicity to otherwise hydrophilic structures, an example being the use of polytetrafluoroethylene (PTFE) or copolymers thereof with hexafluoropropylene or a perfluorovinyl ether. (See Blanchart, U.S. Pat. No. 4,447,505, Yeager, op.cit., and Serpico et al, U.S. Pat. No. 5,677,074.)
In order to achieve durability, uniformity, and structural integrity, it is usually necessary to sinter the fluoropolymers so employed. The fluoropolymers of the art exhibit crystalline melting points well above 200° C., making it is necessary to perform the sintering at temperatures above 300° C. The heating cycle associated therewith is long and complicated. Furthermore, the high temperature tends to degrade other components of the MEA requiring in practice that the sintering take place before the MEA is assembled. A representative sintering cycle is illustrated schematically in FIG. 1.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
Serpico, op.cit., discloses a porous gas diffusion electrode having a catalyst layer containing optimally 15-30% PTFE in a catalyst layer. The resulting catalyst layer is heated to 380° C. in an inert atmosphere prior to combination with the SPE membrane.
MacLeod, U.S. Pat. No. 4,215,183 discloses an electrochemical cell such as a fuel cell comprising an ion exchange membrane electrolyte and catalytic electrodes bonded to the surface of the membrane provided with a wet proofed carbon paper current collector at the oxidizing electrode. The wet-proofed conductor containing 20-35 mg/cm2 of PTFE, is sintered at a temperature of 590-650° F. Further disclosed is a catalyst composition wherein 15-30 wt-% of PTFE particles are intermixed with catalyst particles, which is similarly sintered prior to forming the MEA.
Wilson, U.S. Pat. No. 5,234,777, discloses a gas reaction fuel cell incorporating a thin catalyst layer between a solid polymer electrolyte (SPE) membrane and a porous electrode backing. The catalyst layer is preferably less than about 10 μm in thickness with a carbon supported platinum catalyst loading less than about 0.35 mgPt/cm2. The film is formed as an ink that is spread and cured on a film release blank. The cured film is then transferred to the SPE membrane and hot pressed into the surface to form a catalyst layer having a controlled thickness and catalyst distribution. Alternatively, the catalyst layer is formed by applying a Na+form of a perfluorosulfonate ionomer directly to the membrane, drying the film at a high temperature, and then converting the film back to the protonated form of the ionomer. The layer has adequate gas permeability so that cell performance is not affected and has a density and particle distribution effective to optimize proton access to the catalyst and electronic continuity for electron flow from the half-cell reaction occurring at the catalyst
Fujita et al, Japanese Patent 02007399, discloses a method similar to that of Wilson, op.cit. except PTFE is included in the catalyst composition and the deposition of the catalyst layer to the SPE membrane is effected at 100° C. Under such conditions, the PTFE remains as discrete powder particles within the composition, and is highly susceptible to being flushed out, or simply falling out during use.
Yeager et al, U.S. Pat. No. 4,975,172, discloses gas diffusion electrodes and gas generating or consuming electrochemical cells. The electrode includes an electronically conductive and electrochemically active porous body defining respective gas and electrolyte contacting surfaces, with an ionomeric ionically conductive gas impermeable layer covering the electrolyte contacting surface. The layer includes a layer of a hydrophilic ionic polymer applied directly to the electrolyte contacting surface and a membrane of a hydrophilic ion exchange resin overlying the polymer layer.
The present invention provides for an MEA that is durable, uniform, and possesses good structural integrity, produced by a process which does not require a long, complicated sintering of the fluoropolymers incorporated therein at undesirably high temperatures.